The present invention relates to the refractive-diffractive hybrid lens that has a refractive lens and a diffractive lens structure formed on a lens surface of the refractive lens.
Aberrations of a refractive lens vary as refractive index varies. For an aspherical positive single lens satisfying a sine condition, a focal length shortens and a spherical aberration becomes negative (undercorrected) as a refractive index increases, and the focal length becomes longer and the spherical aberration becomes positive (overcorrected) as the refractive index decreases.
In an optical system of an optical disc apparatus, a change of a focal length of an objective lens due to the change of the refractive index causes no problem because it can be corrected by a focusing mechanism.
However, a change of a spherical aberration causes a problem because a wavefront of a laser beam is disturbed when the spherical aberration is not adequately corrected. If the spherical aberration exceeds an acceptable level, the laser beam can not be converged into the required spot size on an optical disc. In such a case, the optical system can not record or reproduce information data.
The refractive index varies as a temperature changes. In particular, a refractive index of a plastic lens greatly varies as a temperature changes.
Accordingly, the temperature range within which the aberration does not exceed the acceptable level becomes smaller when a plastic lens is employed.
Further, there is a material whose refractive index is not stable during a molding process. Since the refractive index of the material varies depending on a molding condition, the refractive index of the molded lens may be different from the design value of the refractive index, which generates the aberration. Therefore, when the lens is manufactured through the molding process, the lens should not be made from such material.
It is therefore an object of the present invention to provide a refractive-diffractive hybrid lens that resists a temperature change even when it is made from plastic. A further object of the invention is to allow a lens to be made from the material whose refractive index is not stable during a molding process even when the lens is made through the molding process.
For the above object, according to the present invention, there is provided an improved refractive-diffractive hybrid lens, which includes a refractive lens, and a diffractive lens structure having a plurality of concentric ring-shaped steps that are formed on at least one lens surface of the refractive lens. The refractive lens and the diffractive lens structure are designed such that a change of the spherical aberration due to a change of the refractive index becomes small.
With this construction, the change of the spherical aberration can be reduced even when the refractive index varies due to the temperature change. Further, if the refractive index varies during the molding process, the spherical aberration can be kept small. Therefore, when the lens is used as an objective lens of an optical disc apparatus, a laser beam can be converged into the required spot size on an optical disc in spite of the change of the refractive index.
A converging lens such as an objective lens of an optical disc apparatus should satisfy the following conditions.
(a) A spherical aberration is corrected.
(b) A coma is corrected.
These conditions can be satisfied by employing an aspherical surface. The object of the invention is to satisfy the condition (c) in addition to the conditions (a) and (b). (c) A change of spherical aberration due to a variation of a refractive index is reduced.
The aspherical refractive lens cannot satisfy these conditions (a), (b) and (c) at the same time, while it can satisfy the condition (b) and (c). Thus, in the invention, the refractive lens is designed to satisfy the conditions (b) and (c) and the diffractive lens structure is designed to correct the residual spherical aberration, i.e., to satisfy the condition (a). Since the correction effect of the diffractive lens structure does not vary with the refractive index, the spherical aberration of the refractive-diffractive hybrid lens of the invention can be kept small even when the refractive index varies.
The refractive lens may comprise a single lens. When at least one surface of the single lens is an aspherical surface, it is preferable that the positive spherical aberration caused by the refractive lens is counterbalanced with the negative spherical aberration caused by the diffractive lens structure.
An additional optical path length added by the ring-shaped steps of the diffractive lens structure is expressed by an optical path difference function "PHgr"(h) as follows:
"PHgr"(h)=(P2h2+P4h4+P6h6+ . . . )xc3x97mxc3x97xcex
where P2, P4 and P6 are coefficients of second, forth and sixth orders, h is a height from the optical axis, m is a diffraction order and xcex is a working wavelength.
It is preferable that the diffractive lens structure satisfies the following condition (1);
xe2x88x920.20 less than P4xc3x97mxc3x97xcexxc3x97(hMAX/NA)3 less than xe2x88x920.04xe2x80x83xe2x80x83(1) 
where NA is a numerical aperture, and hMAX is a height from the optical axis of a point where the light ray defining the NA intersects the diffractive lens structure.